Previously, there is known a fuel supply system that includes a high pressure pump, which pressurizes relatively low pressure fuel drawn through a fuel pump and supplies the pressurized fuel to high pressure fuel injection valves, while supplying low pressure fuel to, for example, low pressure fuel injection valves. In JP5401369B2 (corresponding to US2012/0312278A1), low pressure fuel is supplied to low pressure fuel injection valves through a fuel chamber of the high pressure pump.
A plunger of the high pressure pump reciprocates even when the fuel is not discharged from the high pressure pump. At this time, the fuel, which is drawn into the pressurizing chamber, is not pressurized and is returned to the fuel chamber. However, when the fuel is not discharged from the high pressure pump, the function of outputting the frictional heat, which is generated through the slide movement of the plunger, along with the fuel discharged from the high pressure pump is lost. Therefore, the fuel, which is returned from the pressurizing chamber to the fuel chamber, is heated to the high temperature by the frictional heat and may possibly contain vapor. Thus, in JP5401369B2 (corresponding to US2012/0312278A1), bubbles, which are accumulated in the fuel chamber, may possibly be outputted along with the low pressure fuel to cause generation of cavitation erosion in a conduit of a supply destination.